Traditionally, metallurgists have wanted to take low quality metals, such as ferrous alloys and low carbon steel, and turn them into high quality steels and more desirable products through inexpensive treatments, including annealing, quenching, and tempering to name a few. Previous attempts have met with limited success in that they did not always produce a desirable product. Other attempts have failed on a large scale due to high processing costs.
Processing of high strength steel generally takes heavy capital equipment expenditures, expensive and dangerous heated fluids, such as quenching oils and quenching salts, and tempering/annealing processes which include the use of ovens, heating equipment, and residual heat from pouring molten steel. These quenching procedures are intended to raise the hardness of the steel to a desirable value. Bainite and martensite are two high strength phases of steel that can be made by these processes and are very desirable materials for certain high strength applications as they generally have Rockwell C hardness of from about 30 and up. The increased hardness correlates to a comparable increase in tensile strength. From widely published charts, it is accepted that a low carbon steel with a Rockwell C hardness of 31 has a tensile strength of about 1005 MPa.
Typical advanced high strength steels include such bainitic and/or martensitic phases. Bainite is generally an acicular steel phase structured of a combination of ferrite and carbide that exhibits considerable toughness with high ductility. Usually formed by austempering, the bainite phase is a very desirable product. One practical advantage of bainitic steels is that relatively high strength levels can be obtained together with adequate ductility without further heat treatment, after the bainite reaction has taken place. Such steels, when made as a low carbon alloy, are readily weldable, and bainite will form in the heat-affected zone adjacent to the weld metal, thereby reducing the incidence of cracking. Furthermore, these steels having a lower carbon content tend to improve the weldability and reduce stresses arising from transformation. When traditional bainite is formed in medium and high carbon steels, weldability is reduced due to the higher carbon content. However, industry would find a great benefit in a high strength steel that is weldable.
The other conventional high strength steel, martensite, is another acicular steel phase made of a hard, supersaturated solid solution of carbon in a body-centered tetragonal lattice of iron. It is generally a metastable transitional structure formed during a phase transformation called a martensitic transformation or shear transformation in which larger workpieces of austenized steel may be quenched to a temperature within the martensite transformation range and held isothermally at that temperature to attain an equalized temperature throughout before cooling to room temperature. In thinner sections, martensite is often quenched in water.
Since chemical processes accelerate at higher temperatures, the strength associated with martensite is easily tempered/destroyed by the application of heat. In some alloys, this effect is reduced by adding elements such as tungsten that interfere with cementite nucleation, but, more often than not, the phenomenon is exploited instead. Since quenching can be difficult to control, most steels are quenched to produce an overabundance of martensite, and then tempered to gradually reduce its concentration until the right structure for the intended application is achieved. Too much martensite leaves steel brittle, whereas too little martensite leaves it soft.
It is a first aspect of the present invention to provide an inexpensive, quick and easy way to produce a low, medium, or high carbon iron-based alloy containing a high percentage of high strength steel while having some of the desirable mechanical properties of traditional bainite and/or martensite.
It is a second aspect of the present invention to provide a method and apparatus for micro-treating low, medium, or high carbon iron-based alloys to contain a desirable quantity of a new microstructure, including coalesced bainite, bainite and/or martensite or bainite itself, martensite itself, ferrite, pearlite, or combinations of the various materials thereof. The micro-treated low, medium, or high carbon iron-based alloy may have varying thicknesses for different applications and may be readily weldable while having high tensile strength, along with the ability to save material and reduce weight.